OpenCode lets people take the Claude-Code-only-API-Key, and lets them use it in a different harness. Anthropic's preferred way for such interaction is getting a different, Claude API key (and not Claude Code SDK API key).
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A rough analogy might be something like getting subsidized drinks from a cafe, provided you sit there a eat food. What if someone says, go to the cafe and get free drink and come sit over at our cafe and order food. It is a loose analogy, but you get the idea.